1. Field of the Invention
The present invention relates to a piston for an internal combustion engine.
2. Description of the Prior Art
A new piston usually has a smaller diameter at a shoulder of a skirt portion than at a lower portion of the skirt portion for the purpose of absorbing the large thermal expansion of the shoulder portion.
However, after repeated impacts against a cylinder bore surface the piston becomes deformed radially inwardly at the lower portion of the skirt portion, as shown in FIG. 15, from configuration A to configuration B. At this time, the piston will impact against the cylinder bore surface at the shoulder portion C of the skirt portion also. Since a clearance between the shoulder portion and the cylinder bore surface is large, impact forces and slapping noise will be increased.
To prevent the clearance between the skirt portion and the cylinder bore surface from increasing, it is useful to increase radial stiffness of the skirt portion. However, if the stiffness of the skirt portion is increased, the shock occurring at the shoulder portion of the skirt portion after the lower portion of the skirt portion finally is deformed will become large, and an oil film formed between the shoulder portion and the cylinder bore surface will rupture to cause scuffing.